Hitherto, apparatuses for controlling reel tension in rolling machine processing lines, rubber or plastic manufacturing equipment, or similar equipment have been constituted by a DC motor,. an electric power converting apparatus and a field power source tension control circuit.
A tension control method of a reel driving motor using a DC motor will be described hereinbelow. A generating torque T.sub.M of the DC motor and a necessary torque T.sub.M ' upon take-up operation are respectively expressed by EQU T.sub.M =K.sub.1 .multidot..phi..multidot.I.sub.a ( 1) EQU T.sub.M '=K.sub.2 .multidot.T.multidot.D (2)
Where I.sub.a is an armature current, .phi. is a field magnetic flux, T is a take-up tension, D is a diameter of a coil, and K.sub.1 and K.sub.2 are constants. The relation among the take-up tension T, field magnetic flux .phi., coil diameter D, and armature current I.sub.a will be represented by ##EQU1## assuming that equations (1) and (2) are equal. On the other hand, a counter-electromotive voltage E of the DC motor is expressed by EQU E=K.sub.3 .multidot..phi..multidot.N (4)
where N is a rotating speed of the motor and K.sub.3 is a constant. In addition, the relation of EQU v=.pi..multidot.D.multidot.N (5)
is satisfied among a take-up speed v, coil diameter D and rotating speed N of the motor
From equations (4) and (5), ##EQU2## is satisfied and from equations (3) and (6), ##EQU3## is satisfied, where K.sub.4 is expressed by ##EQU4##
It will be appreciated from equation (7) that the take-up tension T is proportional to the armature current I.sub.a by making the take-up speed v be proportional to the counter-electromotive voltage E. Namely, the tension control in the reel driving motor using the DC motor is performed by controlling the armature current I.sub.a by making the take-up speed v be proportional to the counter-electromotive voltage E.
Conventionally, various kinds of devices have been made to extend the tension control range; however, all of them fundamentally perform the tandem drive and an example of such a driving method as a prior art is shown in FIG. 2. In this tandem drive, two motors M.sub.1 and M.sub.2 are connected through a clutch 4 and the motors M.sub.1 and M.sub.2 are controlled through motor control circuits 2 and 3 in response to a command from a tension control circuit 1, thereby controlling the reel tension. The two motors M.sub.1 and M.sub.2 are used in case of the high tension control, while the clutch 4 is released and the single motor M.sub.1 is used in case of the low tension control, thereby controlling the tension of a reel 6.
A principle of enlargement of the tension control range due to such a tandem drive will now be described with respect to the cases where the two motors M.sub.1 and M.sub.2 have the same rating and where they have the different ratings.
(1)In the case where the ratings of the motors M.sub.1 and M.sub.2 are the same:
In the case of rolling machines, a range of the armature current I.sub.a which can be accurately set and controlled is generally 1:10 to 1:15 at a current command level. When the setting and controlling range of the armature current I.sub.a is set to 1:10, the setting and controlling ranges of the armature current I.sub.a in the cases where the two motors M.sub.1 and M.sub.2 are coupled and where only the motor M.sub.1 is used will be as follows, if the sum of the rated armature currents when the motors M.sub.1 and M.sub.2 are coupled is 100%.
______________________________________ I.sub.a max I.sub.a min ______________________________________ When the motors M.sub.1 and M.sub.2 100 (%) 10 (%) are connected: When only the motor M.sub.1 50 (%) 5 (%) is used: ______________________________________
Therefore, the setting and controlling range of the armature current I.sub.a becomes EQU 5(%):100(%)=1:20
Thus, it is possible to derive the setting and controlling range of the armature current I.sub.a which is twice that in the case where one motor is used.
(2) In the case where the rating of the motor M.sub.2 is larger than that of the motor M.sub.1 :
Similarly to the foregoing case of (1), the setting and controlling range of the armature current I.sub.a is set to 1:10 and the capacity of the motor M.sub.1 is set to be 1/4 of the capacity of the motor M.sub.2. The setting and controlling ranges of the armature current I.sub.a in the cases where the two motors M.sub.1 and M.sub.2 are coupled and where only the motor M.sub.1 is used will be as follows, if the sum of the rated armature currents when the motors M.sub.1 and M.sub.2 are coupled is 100%.
______________________________________ I.sub.a max I.sub.a min ______________________________________ When the motors M.sub.1 and M.sub.2 100 (%) 10 (%) are connected: When only the motor M.sub.1 25 (%) 2.5 (%) is used: ______________________________________
Therefore, the setting and controlling range of the armature current I.sub.a becomes EQU 2.5(%):100(%)=1:40
Thus, it is possible to obtain the setting and controlling range of the armature current I.sub.a which is four times larger than that in the case where one motor is used.